1. Technical Field
The invention relates to loudspeakers, and more particularly to resonant bending wave speakers of the general kind described in U.S. Pat. No. 6,332,029 (incorporated by reference herein in its entirety). This patent describes a new class of speaker known as a distributed mode loudspeaker (DML).
2. Background Art
It is known from International Application WO97/09846 to provide a loudspeaker comprising two separately driven panels. The first panel is small and designed to operate at higher frequencies than the large second panel in which it is suspended. The frequency ranges of each panel may overlap in the mid-range and a cross-over network may be added to control output in any overlapping frequency range.
It is known from International Application WO98/52381 to have a loudspeaker comprising a larger low frequency panel and a smaller higher frequency panel which are both excited by a common driver. The smaller and larger panels may be attached together by a material forming a controlling compliant coupling whereby differentiation of the high and lower frequency parts of the loudspeaker is achieved.
According to a first aspect of the present invention, there is provided a loudspeaker comprising an assembly of at least two bending wave panel-form acoustic members each having a set of modes which are distributed in frequency, the parameters of at least two of the acoustic members being selected so that the modal distributions of each acoustic member are substantially different and the arrangement being such that the modal distributions of the assembly of acoustic members are interleaved constructively in frequency. The loudspeaker further includes a transducer to apply bending wave energy to the acoustic members to cause them to resonate to produce an acoustic output.
By constructively interleaving the modal distributions of the acoustic members, the overall modal distribution of the loudspeaker is more dense, i.e. has more modes in a given frequency range, than the modal distribution of any individual acoustic member. Thus in contrast to the prior art, the acoustic members are designed to cover substantially overlapping or substantially the same frequency ranges rather than different frequency ranges which may have some overlap in the mid-range (i.e. around 1 to 2 kHz).
In particular the modal distributions may be constructively interleaved whereby the modes in the overall modal distribution of the assembly are more evenly distributed in frequency than the modes of any individual acoustic member. Thus, any xe2x80x9cbunchingxe2x80x9d or clustering of the modes which may be present in an individual acoustic member may be significantly reduced in the overall distribution. The modes in the modal distribution of the assembly may be substantially evenly distributed in frequency. In these ways, the overall output of the loudspeaker may be enhanced and a smoother frequency response may be achieved.
The acoustic members may have different areas and or shapes so that each acoustic member has a different modal distribution as required. Alternatively, different modal distributions may be achieved by using acoustic members which differ in their mechanical parameters, i.e. parameters such as bending stiffness, damping, mass per unit area or Young""s modulus etc.
At least two of the acoustic members may be coupled together by a coupling such that bending wave energy is transmissible between the acoustic members. Thus, the acoustic members may be both mechanically and acoustically coupled by the coupling. In this way, a transducer need only be attached to one face and adjacent faces may be driven by bending wave energy which is transmitted across the coupling. Complex interactions between acoustic members in the assembly, both mechanical and acoustic, may thus be encouraged to increase the excitation of the available modes in each member, particularly if some of the acoustic members are not actively excited.
The assembly of acoustic members may comprise a single piece of stiff lightweight sheet material which should greatly simplify manufacture and assembly. Alternatively, the assembly may comprise a plurality of discrete acoustic members made from stiff lightweight sheet material. A stiff material is one which is self-supporting. The coupling may be sufficiently flexible to allow flat-packing of the assembly. The coupling may be continuous or discontinuous.
For an assembly formed from a single sheet, the coupling may be formed by at least one fold or a parallel pair of folds in the sheet material. A double fold may provide extra compliance and more decoupling between faces. Each fold may be formed by grooving the sheet material and the grooving may comprise local compression of the sheet material.
For an assembly made of discrete members, the coupling may comprise coupling members. The coupling members may comprise hinge portions whereby the acoustic members are moveable relative to one another.
The assembly of acoustic members may form a three-dimensional or box-form loudspeaker which defines a volume, may be of any suitable geometrical shape, e.g. tetrahedron and may be open or closed with different orientations of members. The assembly may comprise a front face and side faces and may be arranged to define a rear opening for example between an opposed pair of rear faces. At least one or two of the acoustic members may be substantially triangular. The assembly may form a truncated pyramid and the plane of the truncation may be angled, for example at 20xc2x0, with respect to the plane of the base of the pyramid.
Alternatively, the acoustic members may be arranged to lie substantially in the same plane. The acoustic members may be in the form of panels which may be flat or curved in one or more planes. For curved panels, the panels may be arranged on the same surface of a volume of rotation.
Each acoustic member may act as a baffle for an adjacent acoustic member. The baffling effect may be improved by partially or completely filling the volume defined by the assembly, e.g. with foam or other known acoustic treatments.
The transducer may comprise an inertial or grounded vibration transducer which may be a moving coil inertial exciter comprising a magnet assembly and a voice coil assembly, a piezoelectric transducer, a magnetostrictive transducer, a bender or torsional transducer (e.g. of the type taught in U.S. patent application Ser. No. 09/384,419 (filed on Aug. 27, 1999)) or a distributed mode transducer (e.g. of the type taught in U.S. patent application Ser. No. 09/768,002 (filed on Jan. 24, 2001)) (each of which is incorporated by reference herein in their entirety). Particularly for folding speakers, the transducers are preferably inertial. The transducers may be mounted to the acoustic members for example as taught in U.S. Pat. No. 6,192,136, U.S. patent application Ser. No. 09/341,295 (filed on Jan. 5, 1998) or U.S. patent application Ser. No. 09/437,792 (filed on Nov. 10, 1999) (each of which is incorporated by reference herein in their entirety) The transducers, particularly low frequency transducers, may be designed to have a fundamental suspension resonance below that of the desired low frequency range of the speaker and a filter may be used to prevent bottoming of the transducers below their fundamental resonance.
The transducer may be a moving coil inertial exciter comprising a magnet assembly and a voice coil assembly. If the transducer is mounted on a sloping face, there is uneven weight loading which may lead to unwanted non-axial movement of the magnet assembly. The magnet assembly may thus be supported in a transducer housing mounted to the acoustic member. The housing may be in the form of a plastic spider which decouples the mass of the transducer from the acoustic member. The transducer housing discourages unwanted non-axial movement of the magnet assembly and hence voice coil damage may be alleviated and the transducer excursion may be limited.
The transducers may comprise respective vibration transducers attached to respective acoustic members. By providing transducers on more than one face, stereo sources may be obtained from a single object. A transducer may be mounted to each face of the box-form structure whereby omnidirectivity at high frequencies may be improved.
Different transducers may be used for different frequency ranges and they may be connected by a crossover, e.g. a first order low pass crossover comprising a series inductor. The filter may comprise a first order series capacitor having a value selected to resonate with the series inductor at a frequency where the output of the speaker as a whole is weak, providing a boost over a controlled frequency band. A passive second order high pass filter may be used to protect the transducer by band-limiting the signal, but may also be used to xe2x80x98ringxe2x80x99 the knee of the filter to obtain boost in the bass, helping to compensate for a dipole gradient roll of or other bass level loss. A modified amplifier transfer function may also be used to boost bass levels.
The stiff lightweight sheet material may be corrugated board or the like. The corrugated board may comprise face skins sandwiching a corrugated core. The assembly may have a front face having a base and at least one side face having a base and the corrugated core may be arranged so that in the front face its corrugations extend perpendicular to the base and/or in the side face its corrugations are at an acute angle to its base.
Alternatively, the stiff lightweight sheet material may be vacuum-formed plastics or extruded twin wall polypropylene sheet, e.g. such as that sold under the trade-mark xe2x80x9cCorrexxe2x80x9d, the latter being generally equivalent to corrugated cardboard. All such materials permit the manufacture of very lightweight, portable, low cost and possible disposable speakers. Alternatively, more durable, long lasting or higher performance sheet materials could be used, e.g. that sold under the trade mark xe2x80x9cTraumalitexe2x80x9d.
Each loudspeaker may have a base and may define a closed box. The loudspeaker may be suspended above the floor and the base may be a radiating acoustic member. Alternatively the base may be defined by the surface on which the loudspeaker stands. The loudspeaker may be mounted on a plinth, a foam or rubber-type strip mounted on the base edge of each acoustic member or on discreet feet or foot-like extensions to the acoustic members themselves. Alternatively, the suspension for the acoustic members may be in the form of a foam or rubber type strip in a moulded groove, a foam or rubber type strip bonded to a surface of the acoustic member or a xe2x80x98wrap aroundxe2x80x99 moulding.
According to another aspect of the invention there is provided a method of making a bending wave panel-form loudspeaker comprising selecting at least two bending wave panel-form acoustic members each having a set of modes which are distributed in frequency, such that the modal distributions of each acoustic member are substantially different and assembling the acoustic members such that the modal distributions of the assembly of acoustic members are interleaved constructively in frequency, and coupling a transducer to the assembly to apply bending wave energy to the acoustic members to cause them to resonate to produce an acoustic output.
The method may comprise making the assembly of acoustic members from a single piece of stiff lightweight sheet material. The acoustic members may be defined in the single piece of sheet material by forming, e.g. by local compression, at least one groove in the sheet material. A parallel pair of grooves may be formed and the grooves may be arranged to enable the sheet material to be folded.
The method may comprise coupling at least two of the acoustic members together such that bending wave energy is transmissible between the acoustic members. The coupling may be such as to allow flat-packing of the assembly.
The stiff lightweight sheet material may be of the kind comprising face skins sandwiching a corrugated core and the assembly may be arranged to define a front face having a base and at least one side face having a base. The corrugated core may be arranged so that in the front face its corrugations extend perpendicular to the base and in the side face its corrugations are at an acute angle to its base.
The set of modes of each acoustic member start from a fundamental or lowest mode and are defined by parameters, including geometry and properties of the material of the acoustic member. The method may thus comprise selecting the parameters of the acoustic members from the group consisting of geometry, size, surface mass density, bending stiffness, internal self damping and anisotropy or isotropy of bending stiffness or thickness. The lowest mode may be determined by the size of the largest individual acoustic member. Accordingly, the size of the largest acoustic member may be selected so that the output of the loudspeaker extends to a desired low frequency limit. The lowest mode of an acoustic member may be selected to be below the fundamental resonant frequency of a transducer coupled thereto, e.g. at least 2 or 3 octaves below. By appropriate parameter selection, acoustic members may have modes as low as 5 Hz and by using a transducer with a fundamental inertial resonance of 40 Hz, the fundamental resonance or whole body bending mode of an acoustic member does not contribute to the acoustic output. Thus, the output may be modally dense and phase decorrelated across the frequency range.
The method may comprise providing a plurality of discrete transducers and selecting them to have different fundamental resonant frequencies. In particular, use of different types of low frequency exciters with different fundamental resonant frequencies will spread the effect of these resonances for the loudspeaker.
In normal operation, a transducer coupled to drive an acoustic member may stiffen the material of the acoustic member directly underneath the transducer coupler. In particular, the circular area of acoustic member enclosed by a voice coil of a moving coil transducer sustains intense tympanic modes which are coherent and remain geometrically organised. The frequency at which this localised resonance occurs is known as the aperture resonance frequency and depends upon the shape of the footprint of the coupler and the properties of the acoustic member. The discrete transducers may be selected to have coupler footprints of different sizes, i.e. different diameter voice coils, such that their respective aperture resonances are at different frequencies. Alternatively, a combination of moving coil and piezo transducers may be used. Each aperture resonance mode may be constructively interleaved with the modal distributions of the acoustic members.
Further features and advantages of the invention, as well as the structure and operation of various embodiments of the invention, are described in detail below with reference to the accompanying drawings.